Pressure transducers employing diaphragms are well known, and they may involve the use of a diaphragm which is either formed of conductive material or has a layer of conductive material on it, and as the diaphragm is shifted in position with changes in pressure, the capacitance between the conductive layer and an adjacent capacitive plate is varied. One widely used capacitive transducer is shown in U.S. Pat. No. 4,388,668, granted June 14, 1983, and assigned to the assignee of the present invention. It has also been proposed to mount thick film or thin film resistors on diaphragms, with the resistance of the film resistors varying as the diaphragms flex and the resistors are stressed.
Differential pressure transducers are also known, and one such transducer is shown in U.S. Pat. No. 4,425,799, granted Jan. 17, 1984 and also assigned to the assignee of the present invention. A differential pressure transducer measures the difference in pressure between two separate bodies of fluid, and in the case of the transducer disclosed in U.S. Pat. No. 4,425,799, this is accomplished by applying pressure to both sides of a diaphragm, with spaced conductive plates being secured to the diaphragm and to an adjacent fixed plate. Accordingly, as the diaphragm changes position under the opposing forces from the two sources of pressure, the capacitance between the two plates varies, and a resultant electrical signal varies to indicate the differential pressure.
The diaphragm may be made of an inert insulating material, such as alumina, which also has a low mechanical hysteresis. In the patents cited hereinabove, the use of either quartz, alumina, or glass such as Pyrex, as the diaphragm and associated fixed plate, are disclosed. Electrically conductive layers are formed on the opposing surfaces of the spaced diaphragm and thicker plate, to provide the capacitance plates of the variable capacitance. When this type of unit is employed as a differential transducer, the fluids to be measured may be applied to both sides of the diaphragm. When this is done, the fluids to be measured, which may be corrosive, would contact the conductive metal plates, and could corrode them. When the electrical plates are corroded, they may engage one another, or may change their output capacitance, both of which would make the transducer unreliable. A similar adverse result could occur from the corrosion of resistors mounted on the diaphragms if they were to be used instead of capacitance sensors.
Accordingly, an important object of the present invention is to provide a simple and reliable differential pressure transducer, and one which is not adversely affected by corrosive fluids, the differential pressure of which is being measured.